Kinetic and spectroscopic characterisation of highly reactive methanesulfonates. Leaving group effects for solvolyses and comments on geminal electronic effects influencing SN1 reactivity

Abstract

Highly reactive methanesulfonates (mesylates, ROMs) have been prepared from 1-phenylethanol, cyclohex-2-en-1-ol, diphenylmethanol and p-methoxybenzyl alcohol by treatment with methane-sulfonyl chloride and triethylamine in dichloro- or trichloro-methane at –20 to 0 °C. The mesylates, characterised in solution by ¹H and ¹³C NMR at –20 °C, were obtained in satisfactory purity (ca. 95 %) in cold solutions but they decomposed by reaction with chloride, triethylamine or the parent alcohol. Rate constants for solvolyses in aqueous acetone and aqueous ethanol have been determined by a fast response conductimetric method. Product selectivities for solvolyses of p-methoxybenzyl mesylate in aqueous ethanol and methanol at 0 °C have been determined by HPLC. From additional new or literature kinetic data for solvolyses of corresponding bromides, chlorides and p-nitrobenzoates (OPNB), Br/Cl, OMs/Br and OMs/OPNB rate ratios were calculated; the results are consistent with electronic effects stabilising the carbocationic transition states and increasing OMs/Br rate ratios for these S_N1 solvolyses; none of the evidence supports a geminal electronic effect on Br/Cl rate ratios (e.g. caused by stabilisation of the initial state in p-methoxybenzyl chloride). Steric effects on ester/halide rate ratios for solvolyses of tertiary substrates are confirmed. Relative rates over a 10¹⁶ range for ester and halide leaving groups are evaluated for solvolyses of 1-phenylethyl substrates in 80 % ethanol–water, updating previous work by Noyce et al. (1972).